U.S. patent number 7,013,135 [Application Number 10/381,697] was granted by the patent office on 2006-03-14 for cell searcher and cell searching method.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Katsuhiko Hiramatsu.
United States Patent |
7,013,135 |
Hiramatsu |
March 14, 2006 |
Cell searcher and cell searching method
Abstract
Peak detector 104 detects the first peak timing where the
correlation value of the first synchronization channel is the
greatest. SSCH identifier 108 identifies the second synchronization
channels by adding the correlation values of the second
synchronization channels at a number of timings, and detects the
timing where the sum of the correlation values of the identified
second synchronization channels is the largest as a second peak
timing. Based on the first and second peak timings, timing
controller 105 instructs SSCH identifier 108 on the timing to
perform correlation calculation. Likewise, based on the first and
second peak timings, frequency corrector 110 calculates the error
in a frame time measured by a base station, and converts this into
a frequency difference, and changes the frequency of an oscillator
in RF receiver 102 in such a way that corrects the frequency
difference. By this means second synchronization channels and
mid-ambles are accurately identified even when a base station gives
a different measurement of a frame time.
Inventors: |
Hiramatsu; Katsuhiko (Yokosuka,
JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JP)
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Family
ID: |
19069967 |
Appl.
No.: |
10/381,697 |
Filed: |
July 25, 2002 |
PCT
Filed: |
July 25, 2002 |
PCT No.: |
PCT/JP02/07531 |
371(c)(1),(2),(4) Date: |
March 27, 2003 |
PCT
Pub. No.: |
WO03/015303 |
PCT
Pub. Date: |
February 20, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040043746 A1 |
Mar 4, 2004 |
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Foreign Application Priority Data
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Aug 7, 2001 [JP] |
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2001-239177 |
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Current U.S.
Class: |
455/423;
375/E1.005; 455/436; 455/456.1 |
Current CPC
Class: |
H04B
1/70735 (20130101); H04B 1/7083 (20130101); H04B
2201/70702 (20130101) |
Current International
Class: |
H04B
1/707 (20060101) |
Field of
Search: |
;455/423,422.1,456,502,436-443,500,501,456.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1117188 |
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Jul 2001 |
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EP |
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11088291 |
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Mar 1999 |
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JP |
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11266181 |
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Sep 1999 |
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JP |
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2000174662 |
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Jun 2000 |
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JP |
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2001168770 |
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Jun 2001 |
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JP |
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Other References
"3GPP TS 25.221 v3.7.0(Jun. 2001)", 3.sup.rd Generation Partnership
Project; Technical Specification Group Radio Access Network;
Physical channels and mapping of transport channels onto physical
channels (TDD), Technical Specification, Release 1999. cited by
other .
"3GPP TS 25.224 V3.7.0(Jun. 2001)". 3.sup.rd Generation Partnership
Project; Technical Specification Group Radio Access Network;
Physical Layer Procedures (TDD), Technical Specification, Release
1999. cited by other.
|
Primary Examiner: Deane, Jr.; William J.
Attorney, Agent or Firm: Stevens, Davis, Miller &
Mosher, LLP
Claims
The invention claimed is:
1. A cell search method of a wireless communication terminal
apparatus, said method comprising: (a) performing a correlation
calculation of a first synchronization channel whereupon a first
peak timing that gives a largest correlation value is detected as a
slot timing; (b) performing correlation calculations of all second
synchronization channels so as to identify a code group assigned to
a cell where the apparatus is located, and detect a timing of a
frame top; and (c) performing a correlation calculation of a
mid-amble belonging to the identified code group, so as to identify
the mid-amble and a scrambling code assigned to the cell where the
apparatus is located; wherein step (b) comprises: identifying a
second synchronization channel at an initial standard timing that
is one frame time after the first peak timing; detecting the timing
where the correlation value of the identified second
synchronization channel is the largest among a plurality of timings
within a predetermined time range centering around the standard
timing; and setting a second and later standard timing at the
timing that is one frame time after a most currently detected
second peak timing, and repeating the second synchronization
channel identification and second peak timing detection by new
standard timings.
2. The cell search method according to claim 1, wherein step (c)
performed after steps (a) and (b) comprises: calculating a
difference of a frame time measured by the apparatus implementing
the cell search method relative to the frame time measured by a
base station apparatus of a communication partner based on the
first and second peak timings, and converting the difference to a
frequency difference; changing an oscillator frequency in such a
way that corrects said frequency; and identifying the mid-amble and
the scrambling code assigned to the cell the apparatus is in.
3. A cell search apparatus comprising: a first correlator that
performs a correlation calculation of a first synchronization
channel; a peak detector that detects a first peak timing where the
first correlator gives a largest correlation calculation result; a
second correlator that performs said correlation calculation of all
second synchronization channels; a timing controller that sets an
initial standard timing one frame time after the first peak timing
and sets a plurality of timings within a predetermined time range
around said initial standard timing; and a second synchronization
channel identifier that compares correlation calculation results in
the second correlator at the initial standard timing and identifies
the second synchronization channels, and detects a second peak
timing among said plurality of timings set by the timing controller
where the identified second synchronization channels give a largest
correlation value, wherein the timing controller sets a second and
later standard timing one frame time after a most currently
detected second peak timing.
4. The cell search apparatus according to claim 3, wherein the
timing controller sets a standard timing, a timing a predetermined
time before said standard timing, and a timing a predetermined time
after said standard timing.
5. The cell search apparatus according to claim 3, wherein the
second synchronization channel identifier identifies a plurality of
second synchronization channels, adds correlation values of said
plurality of second synchronization channels at each timing set by
the timing controller, and detects the second peak timing at a
timing that gives a largest sum of said correlation values.
6. The cell search apparatus according to claim 3, further
comprising a frequency corrector that, with reference to the first
and second peak timings, calculates a difference of a frame time
measured by said cell search apparatus relative to the frame time
measured by a base station apparatus of a communication partner,
converts said difference to a frequency difference, and changes an
oscillator frequency to correct said frequency difference.
7. A wireless communication terminal apparatus comprising a cell
search apparatus, said cell search apparatus comprising: a first
correlator that performs a correlation calculation of a first
synchronization channel; a peak detector that detects a first peak
timing where the first correlator gives a largest correlation
calculation result; a second correlator that performs said
correlation calculation of all second synchronization channels; a
timing controller that sets an initial standard timing one frame
time after the first peak timing and sets a plurality of timings
within a predetermined time range around said initial standard
timing; and a second synchronization channel identifier that
compares correlation calculation results in the second correlator
at the initial standard timing and identifies the second
synchronization channels, and detects a second peak timing among
said plurality of timings set by the timing controller where the
identified second synchronization channels give a largest
correlation value, wherein the timing controller sets a second and
later standard timing one frame time after a most currently
detected second peak timing.
8. A cell search method comprising: (a) performing a correlation
calculation of a first synchronization channel whereupon a first
peak timing that gives a largest correlation value is detected as a
slot timing; (b) performing correlation calculations of all second
synchronization channels so as to identify a code group assigned to
a cell where the apparatus is in, and detect a timing of a frame
top; and (c) performing correlation calculation of a mid-able
belonging to the identified code group, so as to identify the
mid-amble and a scrambling code assigned to the cell where the
apparatus is located; wherein the step (b) comprises: identifying a
second synchronization channel at an initial standard timing that
is one frame time after the first peak timing; detecting the timing
where the correlation value of the identified second
synchronization channel is the largest among a plurality of timings
within a predetermined timing centering around the standard timing;
and setting a second and later standard timing at the timing that
is one frame time after a most currently detected second peak
timing, and repeating the second synchronization channel
identification and second peak timing detection by new standard
timings.
9. The cell search method according to claim 1, wherein step (c)
performed after steps (a) and (b) comprises: calculating a
difference of a frame time measured by the apparatus implementing
the cell search method relative to the frame time measured by a
base station apparatus of a communication partner based on the
first and second peak timings, and converting the difference to a
frequency difference; changing an oscillator frequency in such a
way that corrects said frequency difference; and identifying the
mid-amble and the scrambling code assigned to the cell in which the
apparatus is located.
Description
TECHNICAL FIELD
The present invention relates to apparatus and methods for cell
search, used in mobile communication systems of W-CDMA/TDD
schemes.
BACKGROUND ART
In a mobile communication system, a communication terminal
apparatus, when the power is turned on, searches for a cell to
which it belongs (i.e. initial cell search), and searches for the
cells as it moves across the cells (i.e. idle period cell search).
Below, a cell search method in a mobile communication system of a
W-CDMA/TDD scheme will be explained.
Every cell in a mobile communication system is assigned a
scrambling code and a code group that corresponds to the scrambling
code. As for the code group, there are four combinations of
mid-ambles and scramble codes assigned such that they are not
erroneously identified between neighboring cells.
Moreover, as shown in the control signal frame configuration
diagram of FIG. 1, using predetermined slots in a frame (#0 and #8
in FIG. 1), by the timing t.sub.offset which is offset from the top
of a slot by a predetermined time, a base station apparatus
transmits the first synchronization channel (Primary
Synchronization Code Channel: Cp) that is common to all cells, and
the second synchronization channels (Secondary Synchronization Code
Channel: Cs) that carry three codes to express a code group,
simultaneously. As for the second synchronization channels,
selecting 3 types from 17 types gives 4913=17.sup.3 combinations,
and out of these, the 32 least error detection types are used to
express a code group.
Moreover, a second synchronization channel, Csj (j=1, 2, 3), is
subjected to modulation that is 90.degree..times.n (n=0, 1, 2, 3)
to the phase of the first synchronization channel Cp upon
transmission. b.sub.j in FIG. 1 denotes the phase rotation amount
in each second synchronization channel in relative to the phase of
the first synchronization channel Cp.
As for the cells, these can be selected from Case 1, where
synchronization channels are transmitted using one portion (the kth
slot) of a frame (10 ms), and from Case 2, where synchronization
channels are transmitted using two portions (the kth slot and the
k+8th slot) of a frame (k is a whole number from 0 to 7).
When performing an initial cell search, for the first step, a
communication terminal apparatus performs the correlation
calculation of the first synchronization channel, and detects the
timing giving the largest correlation value (hereinafter "peak
timing") as a slot timing.
Next, for the second step, the communication terminal apparatus
performs the correlation calculation of 17 types of second
synchronization channels and identifies the three types of second
synchronization channels being transmitted from a base station
apparatus. When identifying these second synchronization channels,
the communication terminal apparatus uses 4-frame signals for Case
1, and 2-frame signal for Case 2. Then, the communication terminal
apparatus identifies the code group assigned to its cell based on
the phase rotation amount in four frames of the identified three
types of second synchronization channels and the time offset value
t.sub.offset of the synchronization channels from the top of a
slot, and thus detects the timing of a frame top.
Finally, for the third step, the communication terminal apparatus
performs the correlation calculation of the four types of
mid-ambles belonging to the identified code group, and identifies
the mid-amble and scrambling code assigned to its cell.
Incidentally, to improve the characteristics of this mid-amble
detection, the communication terminal apparatus performs wireless
communication while synchronizing with the base station through AFC
(Automatic Frequency Control).
In a mobile communication system of a W-CDMA/TDD scheme, thus, a
communication terminal apparatus performs an initial cell search
(scrambling code identification) in three steps.
Now, in the above cell search, a communication terminal apparatus
is not frequency-synchronized with a base station, and so the
oscillators in these apparatus have different oscillation
frequencies, thus measuring one frame time differently.
However, since a conventional cell search method does not take the
difference between frame times measured by a communication terminal
and a base station apparatus into consideration, in the second
step, correlation calculation is performed at a wrong timing that
is off the peak timing.
For instance, if the one-frame time measured by a base station
serves as a standard, and, in comparison thereto, the one-frame
time measured by a communication terminal is shorter than that by
the above base station by .alpha.[s], the communication terminal
apparatus, in the second step, develops the difference from the
peak timing by .alpha.[s] for every frame. If second
synchronization channels are identified over four frames, a
difference of maximum 4.alpha.[s] can result. In case this error
develops large (greater than 1/2 chip time, for instance), the
communication terminal apparatus becomes incapable of peak
detection and detecting second synchronization channels.
Moreover, if the difference between the oscillation frequencies of
the oscillators in a communication terminal apparatus and a base
station apparatus grows large, the accuracy of synchronization
detection in the early stages of AFC in the third step decreases,
thereby making accurate mid-amble detection difficult.
DISCLOSURE OF INVENTION
One of the primary objects of the present invention is to provide
apparatus and methods for cell search for use in mobile
communication systems of W-CDMA/TDD schemes that enable accurate
identification of second synchronization channels and mid-ambles
even where a base station gives a different measurement of a frame
time.
The above object can be achieved by, in the second step of a
W-CDMA/TDD scheme-based cell search, comparing the correlation
calculation results of several timings and selecting the second
synchronization channels from the timings that correspond to the
largest correlation calculation results.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a frame configuration diagram of control signal at base
station apparatus;
FIG. 2 is a block diagram showing a configuration of a cell search
apparatus according to the embodiment of the present invention;
and
FIG. 3 shows in detail the second step in a cell search apparatus
according to the embodiment of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with
reference to the accompanying drawings.
FIG. 2 is a block diagram showing a configuration of a cell search
apparatus according to the embodiment of the invention. The
following explanation concerns Case 1, where a frame (10 ms)
transmits synchronization channels using one portion (kth frame)
thereof.
Antenna 101 receives a career frequency signal transmitted from a
base station. RF receiver 102 subjects the signal received by
antenna 101 to various processings including down-conversion,
amplification, and A/D conversion, so as to obtain a baseband
digital signal.
PSCH correlator 103 calculates the correlation between the output
signal from RF receiver 102 and the first synchronization channel.
Peak detector 104 detects the timing where the correlation with the
first synchronization channel becomes maximum (hereinafter "first
peak timing") and outputs the detection result to timing controller
105 and to frequency corrector 110.
The above processings in PSCH correlator 103 through peak detector
104 are the first step of a cell search.
Based on the first peak timing detected in peak detector 104 and
based on the second peak timing detected in SSCH identifier 108,
which will be described later, timing controller 105 controls the
timing to take correlation values out of memories 107.about.107-n.
Each of SSCH correlators 106-1.about.106-n is assigned one of the
second synchronization channels, and calculate the correlation
between the assigned second synchronization channel and the output
signal from RF receiver 102 on a per (1/4) Ts basis. "Ts" denotes
the time of one chip. Memories 107-1.about.107-n store four frames
of the correlation values output from corresponding SSCH
correlators 106-1.about.106-n.
SSCH identifier 108 takes the correlation values out of memories
107-1.about.107-n by the timing instructed by timing controller
105, identifies three second synchronization channels of the same
timing, that correspond to the three largest correlation values,
and outputs these to code group identifier 109. Moreover, as a
second peak timing, SSCH identifier 108 outputs the timing where
the sum value of the identified second synchronization channels
become the largest to timing controller 105 and to frequency
corrector 110. Code group identifier 109 identifies the code group
assigned to its cell based on the phase rotation amount in four
frames of the three second synchronization channels identified in
SSCH identifier 108 relative to the first synchronization channel,
and thus detects the timing of a frame top.
The above processings in timing controller 105 through group
identifier 109 are the second step of a cell search. The above
second step in the cell search apparatus according to the present
embodiment will be described in more details later.
Based on the first peak timing and the second peak timing,
frequency corrector 110 calculates the difference relative to the
frame time measured in the base station, and converts this
calculated difference into a frequency difference. Then, frequency
corrector 110 corrects the frequency of the oscillator in RF
receiver 102 in such a way that corrects the frequency difference.
Incidentally, detailed examples of calculation in frequency
corrector 110 will be described later.
Mid-amble correlators 111-1.about.111-m calculate the correlation
between the mid-amble belonging to the identified code group and
the output signal from RF receiver 102, and outputs the correlation
value to mid-amble identifier 112.
Mid-amble identifier 112 identifies the mid-amble from those
corresponding to the largest correlation values output from
mid-amble correlators 111-1.about.111-n. Moreover, the mid-ambles
and scramble codes are in one-to-one pairs, and in accordance
therewith identifier 112 identifies the cell's scrambling code.
The processings in frequency corrector 110 through mid-amble
identifier 112 are the third step of a cell search. The cell search
apparatus of FIG. 2 completes an initial cell search by
implementing the above first through three steps (i.e. scrambling
code identification).
Next, the second step in the cell search apparatus of the present
embodiment will be described in detail with reference to FIG. 3. In
FIG. 3, frame F.sub.0 denotes the frame where the first peak timing
T.sub.f0 is detected, and frame F.sub.i is the ith frame from frame
F.sub.0.
The window width in timing controller 105 is set in advance to a
certain width (Ts/2 in FIG. 3). Then, timing controller 105
instructs memories 107-1.about.107-n on the timing Tf.sub.1 which
is one frame time after the first peak timing Tf.sub.0, and the
timings (Tf.sub.1-Ts/4) and (Tf.sub.1+Ts/4) that corresponds to the
front and rear ends of the window centering around the timing
Tf.sub.1.
Memories 107-1.about.107-n output the correlation values at the
timings Tf.sub.1, (Tf.sub.1-Ts/4), and (Tf.sub.1+Ts/4) to SSCH
identifier 108.
With frame F1, with reference to the correlation values output from
memories 107-1.about.107-n at Tf.sub.1, SSCH identifier 108
identifies the three largest second synchronization channels.
Moreover, at the timings Tf.sub.1, (Tf.sub.1-Ts/4), and
(Tf.sub.1+Ts/4), SSCH identifier 108 adds the correlation values of
the three identified second synchronization channels, detects the
timing giving the largest sum as a second peak timing, and outputs
the second peak timing to timing controller 105 and to frequency
corrector 110. In the case of FIG. 3, the sum is maximum at the
timing of (Tf.sub.1+Ts/4), and so SSCH identifier 108 sets a second
peak timing on the timing (Tf.sub.1+Ts/4). Incidentally, since the
addition of the correlation values can reduce the impact of noise,
and accordingly, detecting a maximum timing by adding correlation
values and using the result thereof is more accurate than
performing such detection using one correlation value.
That the sum of the correlation values is maximum at the timing
(Tf.sub.1+Ts/4) denotes that the frame time measured in the cell
search apparatus is shorter than that by the base station.
For instance, if a frame time measured in a cell search apparatus
is shorter than that by a base station by .alpha.[s], in case of a
typical cell search apparatus, the timing difference in correlation
calculation becomes 2.alpha.[s] in frame F.sub.2. In contrast, if a
cell search apparatus according to the present invention is used,
the timing difference upon correlation calculation in frame F.sub.2
becomes (2.alpha.-Ts/4) [s], and the difference is thus reduced
compared to the above typical cell search apparatus.
Timing controller 105 instructs memories 107-1.about.107-n on the
timing T.sub.f2, and the timings (T.sub.f2-Ts/4) and
(T.sub.f2+Ts/4) corresponding to the front and rear ends of the
window centering around the timing T.sub.f2.
Memories 107-1.about.107-n output the correlation values at
T.sub.f2, (T.sub.f2-Ts/4), and (T.sub.f2+Ts/4) to SSCH identifier
108.
With frame F2, with reference to the correlation values output from
memories 107-1.about.107-n at the timing of Tf.sub.2, SSCH
identifier 108 identifies the three largest second synchronization
channels. Moreover, at the timings Tf.sub.2, (Tf.sub.2-Ts/4), and
(Tf.sub.2+Ts/4), SSCH identifier 108 adds the correlation values of
the three identified second synchronization channels, and outputs
the timing giving the largest sum as another second timing to
timing controller 105 and frequency corrector 110. In the case of
FIG. 3, the sum is maximum at the timing of Tf.sub.2, and so SSCH
identifier 108 sets another second peak timing on the timing
Tf.sub.2. Thereafter, with frames F.sub.3 and F.sub.4, the cell
search apparatus identifies the second synchronization channels in
the same manner as in the processings for frame F.sub.2.
Comparison of the correlation values at several timings including
the standard timing and the setting of a new standard timing upon
the timing that gives the largest correlation value thus makes it
possible to save timing differences upon correlation calculation
within a given range, so that peak detection and second
synchronization channel identification in the second step can be
performed with certainty.
Incidentally, in the above explanation, the window width is set
centering around a standard timing that is one frame time behind
the first or the second peak timing, and, with respect to the three
timings including the standard timing and the timings corresponding
to the front and rear ends of the window, the sums of the
correlation values are compared. However, the present invention is
by no means limited to the above and the comparison of the sums of
correlation values can be performed with respect to a number of
timings within a predetermined time range centering around a
standard timing.
Next, the method of calculation in frequency corrector 110 will be
described in detail assuming the case of FIG. 3.
In FIG. 3, the second peak timings in the frames are respectively
(Tf.sub.1+Ts/4), Tf.sub.2, (Tf.sub.3+Ts/4), and Tf.sub.4,
indicating that error of Ts/2 has occurred over four frames. Where
one frame is 10 ms and Ts is (1/3.84).times.10.sup.-6 ms, this
gives error of (1/(4.times.2.times.3.84)).times.10.sup.-6 ms per
frame. If this is converted into frequency difference, using ppm
(10.sup.-6) for the frequency difference unit, the result will be
(1/(4.times.2.times.3.84)) ppm. Such calculation is performed in
frequency corrector 110, and the frequency of the oscillator in RF
receiver 102 is corrected in such a way that corrects the frequency
difference. Incidentally, given that voltage control oscillators
are in common use, frequency corrector 110 supplies voltage that is
equivalent of the frequency difference to the oscillator.
By correcting the frequency difference obtained thus and setting
the initial level of AFC, the accuracy of detection in the early
stages of AFC can be improved, so as to enable accurate mid-amble
detection.
Furthermore, referring to the third step, receiving second
synchronization channels that are not used for AFC alone increases
power consumption. In contrast, the present embodiment solves the
above problem as second synchronization channels become unnecessary
during AFC in the third step.
As obvious from the above description, according to the present
invention, the differences between the timings of correlation
calculation can be constantly saved within a given range, so that
second synchronization channels and mid-ambles can be accurately
identified.
The present application is based on Japanese Patent Application No.
2001-239177 filed on Aug. 7, 2001, the entire content of which is
expressly incorporated herein by reference.
INDUSTRIAL APPLICABILITY
The present invention is suitable for use with wireless
communication terminal apparatus in mobile communication systems of
W-CDMA/TDD schemes.
* * * * *